The development of improved typhoid vaccines to prevent antibiotic-resistant typhoid fever in developing countries is a high global public health priority. The potential use of S. Typhi as a bioterror agent has added an additional reason to study this important global pathogen. The goal of this project is to characterize, at the mucosal and systemic levels, both short and long-term humoral and cell-mediated immunity (CMI) induced by oral immunization with the licensed attenuated S. Typhi Ty21a vaccine and to explore the interactions between Ty21a immunization and the gut microbiota. The development of improved live oral typhoid vaccines has been hampered by a lack of detailed information of the specific determinants of protective immunity to S. Typhi infection. Moreover, the effector and memory immune responses to S. Typhi in circulation and in the gut microenvironment have not been characterized. Since the gastrointestinal tract is the site of entry of S. Typhi, the presence of effective and sustained immune responses in the local microenvironment are likely to be pivotal in protection from infection. Results from studies in typhoid patients and vaccine trials with attenuated S. Typhi strains in adults suggest that antibodies (Ab) to common S. Typhi antigens appear to play a protective role against S. Typhi infection, however, it is not known if such Abs mediate protection or serve as a surrogate for the more dominant protective cellular mediated immunity (CMI) resulting in the elimination of this iritracellular S. Typhi. In spite of the fact that children stand to benefit the most from improved typhoid vaccines, only cursory information is available concerning serum Ab levels and, to our knowledge, there are no reports on the induction of antibody secreting cells (ASC), memory B cells (BM) or CMI in this group of individuals. The elderly represent another population group that is understudied in terms of the generation of immune responses to this oral vaccine, as no information is available concerning the induction of either Ab or CMI responses. The growing awareness of the importance of the gut microbiota in health and disease has also raised the question as to the influence of the complex community of microorganisms that inhabit the gastrointestinal tract in oral immunization and vice versa. Therefore, this application in addition to examining the immune response, will also examine the interactions between immune responses to Ty21a immunization and the gut microbiota in children, adults and the elderly. Overall this application addresses two important aspects of the development of immunity to S. Typhi.